I. Field of the Invention
The present invention relates to wireless communication systems and devices. More particularly, the present invention relates to a novel and improved method and system for rapidly and automatically switching between wireless communication systems in a multiple-mode wireless communication device.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in a wireless communication system in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. An example of a TDMA communication system is the pan-European Global System for Mobile Communications (GSM). An example of an analog FDMA system is the Advanced Mobile Phone System (AMPS) presently used in the U.S. for cellular communications.
However, the spread spectrum modulation technique of CDMA has significant advantages over these other techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, issued Mar. 31, 1992, entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM", and U.S. Pat. No. 5,109,390, issued Apr. 28, 1992, entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", both assigned to the assignee of the present invention and incorporated by reference herein.
The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for cell-site and mobile unit power control is disclosed in U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", Ser. No. 07/433,031, filed Nov. 7, 1989, also assigned to the assignee of the present invention. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
In the field of wireless communications, such as cellular, wireless local loop, and Personal Communication Services (PCS), base stations communicate with remote subscriber units such as portable radiotelephones. For simplicity, the term "mobile station" will be used herein to refer to such remote subscriber units, although it is understood that some remote subscriber units, such as wireless local loop phones, do not often move throughout the wireless environment, but rather are generally stationary.
Typically, in any geographic service area, there will be more than one wireless communication service provider. For example, in cellular systems in the United States, there are usually two service providers, one whose system is designated System "A", and another whose system is designated System "B". For the currently planned PCS services in the United States, there are many more service providers, denoted by blocks "A"-"F", covering the same geographical service area. The available frequency spectrum for each geographical service area is divided up among these wireless telecommunication service providers. Each service provider typically operates its own base stations and other network equipment.
In accordance with the various wireless communication standards, including Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) Interim Standard IS-95, entitled "Mobile Station - Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", there exists dual-mode CDMA/AMPS portable radiotelephones which may communicate with either a CDMA base station or an AMPS base station. Furthermore, there are other industry standards existing or in development which provide for dual-mode operation among other modulation and multiplexing schemes such as dual-mode CDMA PCS band and AMPS, dual-mode CDMA PCS band and CDMA cellular band, dual-mode GSM and AMPS, and various other combinations of the known modulation and multiplexing schemes.
As can easily be appreciated from the number of co-located service providers and the number of communication protocols that may be used, there are a large number of possible communication systems that may be operating in a single geographic area, each with varying degrees of coverage. For example, since AMPS was the first analog FM-based cellular system to gain wide market acceptance in the United States, AMPS communications systems presently provide nearly 100% coverage for the entire populated regions of the U.S. However, as other competing communication systems such as CDMA cellular and CDMA PCS systems are being deployed, their total coverage area is rapidly expanding. Thus, it is anticipated that there will be many co-located communication systems with varying and overlapping degrees of coverage.
Due to the advantages of CDMA as outlined above, many users of dual-mode CDMA/AMPS mobile stations prefer to use CDMA service whenever it is available and use AMPS service only when CDMA service is not available. Furthermore, a particular user of a dual-mode CDMA cellular and CDMA PCS portable radiotelephone may prefer to use the PCS services over the cellular services for various reasons. For these reasons, dual-mode mobile stations designed in accordance with IS-95 generally allow the user to select a preferred mode of operation (i.e. CDMA or AMPS), and the mobile station will operate accordingly. Other standards may allow the same userpreferences, or may "hard-code" a particular system priority.
In any event, whenever the user of a dual-mode mobile station is in a geographical area where good coverage exists for one of the non-preferred communication systems, for example AMPS, but imperfect coverage for the preferred system, for example CDMA, the user will generally wish to make a "seamless" transition between the two systems without requiring excessive attention to the portable radiotelephone.
In the art, the forward link (base station to mobile station) is generally assumed to fade together with the reverse link (mobile station to base station). However, due to the complexities of network planning, the forward and reverse links may be somewhat imbalanced. That is to say that the forward link may not be receivable by the mobile station in some localized areas where a base station should otherwise be able to receive the reverse link. This first case is generally referred to as being "forward link limited." A forward link limited condition might be caused by an obstruction or reflection in the path of the forward link that might not be as harmful to the reverse link due to the difference in frequency between the forward and reverse links. Conversely, the forward link may be strong enough that it is receivable by the mobile station in some localized areas where the base station is not able to receive the reverse link. This latter case is generally referred to as being "reverse link limited." A reverse link limited condition might also be caused by differences in propagation path, or perhaps because the service provider has increased the effective radiated power (ERP) of the base station in order to fight jamming by other nearby base stations belonging to a competing system.
To illustrate the undesirable effects of a forward link limited condition, consider the case where a dual-mode CDMA/AMPS mobile station is operating in the CDMA mode, and moves into a deep fade area for the forward link of the CDMA system, such as into a steep valley, and thereby loses the CDMA forward link (i.e. can no longer receive and demodulate the pilot signal from the base station), but a sufficient forward link signal from a nearby AMPS system is still present. In a conventional dual-mode mobile station, if the user attempts to originate a call during this deep fade, the call will be unsuccessful and a "No Service" and "Call Failed" indication will be displayed to the user while the dual-mode mobile station attempts to re-acquire service. After reacquiring service, the user of the conventional dual-mode mobile station will be required to re-initiate the call origination by redialing the number.
As to the reverse link limited condition, consider the case where the dual-mode CDMA/AMPS mobile station is operating in the CDMA mode, and moves into a deep fade area for the CDMA reverse link, such as inside a building or near the edge of a cell's coverage, and thereby is unable to transmit any messages to the CDMA base station successfully, but would be able to successfully transmit to an AMPS base station if it were operating in the AMPS mode. Further assume that the CDMA forward link is still strong enough in this case to be successfully demodulated by the dual-mode mobile station. In a conventional dual-mode mobile station, a signal strength indication such as a number of signal bars would be displayed on the mobile station's visual display because of the strength of the forward link. However, if the user attempts to originate a call during this condition, the call will be unsuccessful and a "Call Failed" indication will be displayed to the user. Thus, the user will see a satisfactory service indication on the display due to the relative strength of the forward link, but will still be unable to communicate with the CDMA base station.
In this reverse link limited situation, the dual-mode mobile station would be unable to acknowledge a page, originate a call, or even register with the CDMA base station even though a satisfactory service indication is displayed. Furthermore, in a conventional dual-mode mobile station, since the mobile station can successfully demodulate the preferred system (CDMA), it would not attempt to acquire the non-preferred system (AMPS), and thus the user would be unable to originate or receive any calls unless the user manually took action to force the phone to acquire the non-preferred system, such as by changing the mobile station to an "AMPS only" mode and then redialing the outgoing phone number. Also, the user would be unaware of the reverse link limited condition because the conventional dual-mode mobile station would indicate satisfactory service.
Thus, the conventional dual-mode mobile station under both forward link limited and reverse link limited conditions requires the user at least to take some affirmative action to redial a phone number due to a failed call attempt, and at worst requires the user to access a user-preferences menu to manually switch the mobile station to an alternate system. What is needed is a mobile station that automatically and seamlessly switches to an alternate, available system when it encounters a forward link limited condition or a reverse link limited condition, without requiring the user to take any affirmative actions to select the alternate system or re-initiate a call origination.